1. I'm biased toward Balmung personally buuuut you can pick whichever you think sounds better, or give each a try! Mateus is smaller, but being that it's newer and has fresh people flowing to it, it has lots of opportunities. If you like smaller and more intimate communities, I'd recommend Mateus. Balmung, however, has a larger population and is a safe bet to always have a flourishing RP community whereas Mateus's future is a bit more rocky if and when Balmung is open again. If you like the stability and having more RPers/events/groups to choose from, I'd recommend Balmung. Even old role-players shouldn't mind you being new, and can hopefully help show you the ropes, so I wouldn't worry about trying to be around other new RPers (though they're also helpful in that you can learn and take the plunge alongside them).
2. You don't have to have your character speak in archaic English, plenty of people RP their characters speaking in modern English. The only thing I'd suggest is to avoid any slang that is too modern and wouldn't make sense in the setting. A character speaking in modern English is definitely less distracting imo than someone trying to write in Ye Olde English and not really knowing how to do it correctly so go with whatever makes you the most comfortable!! (I would recommend learning what you can of older English and Eorzean terminology just so you can understand other characters/RPers but you can always ask OOC if any meaning is unclear!)
3. You don't have you have your character's entire backstory thought out before you jump into RP, so I wouldn't fret it! For me, I just get a base idea of my character's appearance, background, personality, mannerisms, goals, etc. and jump into RP. From there, I fill out the bio as I go along with whatever traits the character has adapted via RP or whatever details I find I've had to make up on the fly in role-play. Of course this is just want I find best for me, some people like to have everything fleshed out first, but it's definitely not necessary.
4. You don't have to write out any dialect/accents, that's just a matter of personal preference! Some people think it gives the RP flavor and can easily communicate their characters origins and make their "voice" easier to hear in your head. Some people think it's distracting or difficult to read at times, and there's a real world argument that accents should not be written out phonetically in dialogue because it implies there is one "correct" accent and anything else is "wrong." So it's definitely just personal choice!
5. The classes' and jobs' in game abilities often tend to be a bit more flashy and/or overpowered for a more "realistic" style of combat, if that's what you prefer. Some people like a "fantasy" approach and like to incorporate the flashier or less believable skills, and that's totally fine. I'd say just try to figure out which you'd prefer. You don't have to actually copy/use the actual class skills in your RP. If your character is, for example, a rogue as was mentioned, in combat you could have your character do whatever you think they realistically could do with a pair of knives. You can add in moves and tactics that aren't featured in in-game skills, and you can also opt not to use some moves featured in game skills if you think they're too far out there as far as believability goes.
2. You don't have to have your character speak in archaic English, plenty of people RP their characters speaking in modern English. The only thing I'd suggest is to avoid any slang that is too modern and wouldn't make sense in the setting. A character speaking in modern English is definitely less distracting imo than someone trying to write in Ye Olde English and not really knowing how to do it correctly so go with whatever makes you the most comfortable!! (I would recommend learning what you can of older English and Eorzean terminology just so you can understand other characters/RPers but you can always ask OOC if any meaning is unclear!)
3. You don't have you have your character's entire backstory thought out before you jump into RP, so I wouldn't fret it! For me, I just get a base idea of my character's appearance, background, personality, mannerisms, goals, etc. and jump into RP. From there, I fill out the bio as I go along with whatever traits the character has adapted via RP or whatever details I find I've had to make up on the fly in role-play. Of course this is just want I find best for me, some people like to have everything fleshed out first, but it's definitely not necessary.
4. You don't have to write out any dialect/accents, that's just a matter of personal preference! Some people think it gives the RP flavor and can easily communicate their characters origins and make their "voice" easier to hear in your head. Some people think it's distracting or difficult to read at times, and there's a real world argument that accents should not be written out phonetically in dialogue because it implies there is one "correct" accent and anything else is "wrong." So it's definitely just personal choice!
5. The classes' and jobs' in game abilities often tend to be a bit more flashy and/or overpowered for a more "realistic" style of combat, if that's what you prefer. Some people like a "fantasy" approach and like to incorporate the flashier or less believable skills, and that's totally fine. I'd say just try to figure out which you'd prefer. You don't have to actually copy/use the actual class skills in your RP. If your character is, for example, a rogue as was mentioned, in combat you could have your character do whatever you think they realistically could do with a pair of knives. You can add in moves and tactics that aren't featured in in-game skills, and you can also opt not to use some moves featured in game skills if you think they're too far out there as far as believability goes.